Instant Pot French Onion Soup

Name: Instant Pot French Onion Soup
Ingredients: 2 tbsp. butter; 2 tbsp. olive oil; 3 lb. yellow onions, thinly sliced; 3 cloves garlic, chopped; 1 tbsp. granulated sugar; 6 sprigs of thyme, plus more leaves for garnish; 3 tbsp. all-purpose flour; 1/2 c. dry white wine; 6 c. beef stock; 2 tbsp. Worcestershire sauce; 1 tsp. kosher salt; 1/2 tsp. ground black pepper, plus more for serving; 1 bay leaf; 6 slices french bread, lightly toasted; 1 tbsp. dijon mustard; 8 oz. shredded gruyere or swiss cheese